1. Field of the Invention
The present invention relates to a scaling process for a virtual object in a virtual space.
2. Related Background Art
For a three-dimensional computer graphics (hereinafter described as 3DCG) system for generating an image of an object in a three-dimensional space projected on a two-dimensional plane, techniques are known which magnify/reduce (scale) the size of an object as desired by using a predetermined point in the three-dimensional space as a center.
In such a 3DCG system, two methods of setting a center position of scaling are known:                (1) the center position is set to a predetermined fixed position; and        (2) a system operator interactively sets the center position by looking at a projected image of an object.        
In the former method, the scaling center is set to the origin of a coordinate system specific to an object. In the latter method, an apex of a cube circumscribing an object is designated on a projected image with a pointing device, and this apex is used as the scaling center.
In both the methods, since the scaling center is determined irrelevant to a viewpoint, a projected image of the object becomes sometimes not suitable for the observation of the object, depending upon the position of the scaling center.
FIG. 8 is a diagram showing an example of such problems. In FIG. 8, reference numeral 801 represents a virtual object before magnification, reference numeral 802 represents a viewpoint from which the object is observed, and reference numeral 803 represents a scaling center. If the object 801 is magnified uniformly in all directions around the scaling center 803, the object 801 is changed to an object 804. It is assumed that this system aims to observe the perspective of the object 801 or 804.
As shown in FIG. 8, before the scaling, the viewpoint 802 is outside the object 801 and the perspective of the object can be observed in a projected image 805.
However, after the scaling, the viewpoint 802 is inside the object 804 so that a projected image 806 shows the inside of the object and the system objective of observing the perspective is not achieved. Depending upon setting of a scaling center, there is associated with a problem that the viewpoint from which an object is observed is positioned inside the object. This problem is called a “first problem.”
FIG. 9 is a diagram showing another example of the problems. In FIG. 9, reference numeral 901 represents a virtual object before magnification, reference numeral 902 represents a viewpoint from which the object is observed, and reference numeral 903 represents a scaling center. If the object 901 is magnified uniformly in all directions around the scaling center 903, the object 901 is changed to an object 904. It is assumed that this system aims to observe the perspective of the object 901 or 904. Before the scaling, an image of the object 901 observed from the viewpoint 902 is an image 905 and the perspective of the object can be observed.
However, after the scaling, the object moves away from the viewpoint so that only a portion of the object can be observed as a projected image 906 or the size of a projected image becomes very small and the, details are difficult to be observed. Depending upon setting of a scaling center, there is associated with a problem that an object to be observed moves away from the viewpoint. This problem is called a “second problem.”